A ΔΣ-type A/D converter utilizing a voltage-controlled oscillator (VCO) as an integrator has been proposed. This type of A/D converter has nonlinear characteristics in frequency change to a VCO voltage, which is a cause of resolution degradation. In order to solve this problem, techniques of calibrating the frequency nonlinearity by voltage sweeping, mitigating the nonlinearity using a D/A converter in a VCO negative-feedback loop, etc. have been proposed.
However, these improved techniques have a problem in that the A/D converter cannot operate rapidly and, in addition, power consumption and circuit area may increase.